This invention relates to a process for the treatment of crude oils and, more particularly, to the hydroconversion of heavy hydrocarbons in the presence of additives and catalysts to provide useable products and further prepare feedstock for further refining.
As the reserves of conventional crude oils decline, heavy oils must be upgraded to meet world demands. In heavy oil upgrading, heavier materials are converted to lighter fractions and most of the sulfur, nitrogen and metals must be removed. Heavy oils include materials such as petroleum crude oil, atmospheric tower bottoms products, vacuum tower bottoms products, heavy cycle oils, shale oils, coal derived liquids, crude oil residuum, topped crude oils and the heavy bituminous oils extracted from oil sands. Of particular interest are the oils extracted from oil sands and which contain wide boiling range materials from naphthas through kerosene, gas oil, pitch, etc., and which contain a large portion of material boiling above 524° C. These heavy hydrocarbon feedstocks may be characterized by low reactivity in visbreaking, high coking tendency, poor susceptibility to hydrocracking and difficulties in distillation. Most residual oil feedstocks which are to be upgraded contain some level of asphaltenes which are typically understood to be heptane insoluble compounds as determined by ASTM D3279 or ASTM D6560. Asphaltenes are high molecular weight compounds containing heteroatoms which impart polarity.
Heavy oils must be upgraded in a primary upgrading unit before it can be further processed into useable products. Primary upgrading units known in the art include, but are not restricted to, coking processes, such as delayed or fluidized coking, and hydrogen addition processes such as ebullated bed or slurry hydrocracking (SHC). As an example, the yield of liquid products, at room temperature, from the coking of some Canadian bitumens is typically about 55 to 60 wt-% with substantial amounts of coke as by-product. On similar feeds, ebullated bed hydrocracking typically produces liquid yields of 50 to 55 wt-%. U.S. Pat. No. 5,755,955 describes a SHC process which has been found to provide liquid yields of 75 to 80 wt-% with much reduced coke formation through the use of additives.
In SHC, a three-phase mixture of heavy liquid oil feed cracks in the presence of gaseous hydrogen over solid catalyst to produce lighter products under pressure at an elevated temperature. Iron sulfate has been disclosed as an SHC catalyst, for example, in U.S. Pat. No. 5,755,955. Iron sulfate monohydrate is typically ground down to smaller size for better dispersion and facilitation of mass transfer. Iron sulfate (FeSO4) usually requires careful thermal treatment in air to remove water from iron sulfate which is typically provided in a hydrated form. Water can inhibit conversion of FeSO4 to iron sulfide and typically must be removed. It is thought that iron sulfate monohydrate decomposes slowly in an SHC to form iron sulfide. Drying the iron sulfate monohydrate in-situ initially dehydrates to FeSO4 as shown in Formula (1). However, FeSO4 also rehydrates to the monohydrate during its decomposition to form iron sulfide in Formula (2). Ultimately, FeSO4 converts to iron sulfide as shown in Formula (3):2Fe(SO4).H2O+8H2→2Fe(SO4)+2H2O+8H2  (1)2Fe(SO4)+2H2O+8H2→FeS+Fe(SO4).H2O+4H2O+4H2  (2)FeS+Fe(SO4).H2O+4H2O+4H2→2FeS+10H2O  (3)Consequently, the amount of water in the system may limit the rate at which iron sulfide can form. Thermal treatment also removes volatiles such as carbon dioxide to make the catalyst denser and opens up the pores in the catalyst to make it more active.
Iron sulfate already contains sulfur. The thermal treatment converts the iron in iron sulfate to catalytically active iron sulfide. The sulfur from iron sulfate contributes to the sulfur in the product that has to be removed. Other iron containing catalysts such as limonite, which contains FeO(OH).nH2O, require presulfide treatment for better dispersion and conversion of the iron oxide to the active iron sulfide according to CA 2,426,374. Presulfide treatment adds sulfur to the catalyst and consequently to the heavy hydrocarbon being processed. As such, extra sulfur must usually be removed from the product. The active iron in the +2 oxidation state in the iron sulfide catalyst is required to obtain adequate conversion and selectivity to useful liquids and to avoid higher coke formation. U.S. Pat. No. 4,591,426 mentions bauxite without examining it and exemplifies limonite and laterite as catalysts. SHC catalysts are typically ground to a very small particle diameter to facilitate dispersion and promote mass transfer.
During an SHC reaction, it is important to minimize coking. It has been shown by the model of Pfeiffer and Saal, PHYS. CHEM. 44, 139 (1940), that asphaltenes are surrounded by a layer of resins, or polar aromatics which stabilize them in colloidal suspension. In the absence of polar aromatics, or if polar aromatics are diluted by paraffinic molecules or are converted to lighter paraffinic and aromatic materials, these asphaltenes can self-associate, or flocculate to form larger molecules, generate a mesophase and precipitate out of solution to form coke.
Toluene can be used as a solvent to dissolve to separate carbonaceous solids from lighter hydrocarbons in the SHC product. The solids not dissolved by toluene include catalyst and toluene insoluble organic residue (TIOR). TIOR includes coke and mesophase and is heavier and less soluble than asphaltenes which are soluble in heptane. Mesophase formation is a critical reaction constraint in slurry hydrocracking reactions. Mesophase is a semi-crystalline carbonaceous material defined as round, anisotropic particles present in pitch boiling above 524° C. The presence of mesophase can serve as a warning that operating conditions are too severe in an SHC and that coke formation is likely to occur under prevailing conditions.